Capacity intruder alarm having capacitive a.c. coupling and d.c. bias coupling in parallel between a detector and amplifier



p 1956 E. G. QUEST ETAL 3,276,005

CAPACITY INTRUDER ALARM HAVING CAPACITIVE A.C. COUPLING AND D.C. BIASCOUPLING IN PARALLEL BETWEEN A DETECTOR AND AMPLIFIER Filed May 5, 1964QAJIN .II

INVENTORS United. States Patent CAPACITY rNTRUonii AIZARM HAVING CAPACI-TIVE A.C. COUPLING AND D-C. BIAS COUPLING IN PARALLEL BETWEEN A DETECTORAND AMPLIFIER Eric G. Quist, Roxbury, Conn, and Richard H. Geis,Hopewell Junction, N.Y., assignors to Mosler Research Products, Inc.,Danbury, Conn, a corporation of Delaware Filed May 5, 1964, Ser. No.365,034 7 Claims. (Cl. 340258) This invention relates to security alarmsystems and is particularly directed to a novel alarm system of thecapacity type.

The present capacity alarm is particularly adapted to protect smallareas against unauthorized intrusion. For example, the alarm can beutilized to protect the ofiice area surrounding file cabinets, safes andthe like. In general, the present alarm system comprises an oscillatorwhich is connected to a tuned resonant circuit. The tuned resonantcircuit includes an antenna element which may be constituted in part bythe files, safes and other elements being protected. The alarm furtherincludes a detector for sensing any changes in capacity in the resonantcircuit, such as might be caused by the presence of an intruder in thearea. This change in capacity manifests itself as a change in potentialof the output of the detector. In the present alarm system the detectoris coupled to an amplifier stage which in turn controls the energizationof a relay for actuating either a remote alarm, a local alarm or acombination of the two.

It will be appreciated by those skilled in the art that the presence ofan intruders body in the electromagnetic field surrounding the antennaresults in a relatively small change in capacitance, for example, achange of the order of micromicrofarads. Consequently, the detector andrelay amplifier must be quite sensitive. At the same time, while thealarm system must signal the approach of an intruder with the utmostreliability, it is likewise important that the system not give falsealarms due to slowly changing environmental conditions such astemperature, humidity and the like. Nevertheless, if these slow changesin the detector output continue beyond a given amount, they indicate afailure in one or more of the alarm components, such as battery voltageor the like. In such a case, the alarm system is not efifective toprovide proper surveillance of the area. Consequently, it is desiredthat an alarm signal be given if the detector output change exceeds apredetermined amount, no matter how slowly this change takes place.

The present invention is predicated upon the concept of providing afail-safe circuit eifective to cause an alarm under each of these twoconditions: a rapid change in detector output corresponding to anintrusion and a change in output in excess of a predetermined amountcorresponding to a malfunction. More particularly, the present failsafesystem interconnects the detector stage and the amplifier stage of thealarm. The fail-safe circuit comprises a first purely resistive pathbetween the detector and the amplifier, which path provides an operatingbias for the amplifier. The fail-safe circuit further comprises a secondconductive path including a capacitor and a series connected sensitivityresistor interconnecting the detector and amplifier stage. The amplifieris rendered normally conductive by the applied bias and is connected inthe alarm circuit so that the alarm is actuated when the output of theamplifier stage is substantially reduced or is completely cut off.

The first conductive path of the fail-safe circuit, which provides anoperating bias for the amplifier, protects the system against componentfailure no matter how slowly Patented Sept. 27, 1966 that componentfailure may cause a change in detector voltage. For if the oscillator,tuned resonant circuit or detector are not functioning properly, thedetector output will not be sufiicient to apply the requisite biaspotential to the amplifier and this amplifier will in turn cease toconduct sufiicient current to prevent actuation of the alarm.

The second conductive path includes a capacitor which is charged up whenthe alarm unit is first turned on. When a sudden increase occurs in thecapacitance of the resonant circuit due to the intrusion of a burglar,or the like, the detector potential drops rapidly and the capacitordischarges in the reverse direction to substantially decrease or stopthe output of the amplifier and thereby actuate the alarm.

One important advantage of the fail-safe circuit is that it providesmaximum protection against both the intr-usion of the area andmalfunction of the unit with very simple and reliable circuitry.

Still another advantage of the present alarm system is that it minimizesthe time during which the system is insensitive after it is turned on.More particularly, it Will be appreciated that most alarm systems areutilized only during portions of each twenty-four hour period, forexample, at night when an office is empty. Thus, a typical alarm isturned 01f in the morning when the ofli-ce is opened and workers arriveand is turned on again at night when the last worker leaves. In thepast, alarm systems of the capacitor type have been subject to thedefect that for an appreciable length of time after the alarm was turnedon it was relatively insensitive so that an intrusion could occurwithout setting off an alarm.

The present invention is in part based upon the concept of minimizing,or substantially eliminating, this period of insensitivity by providinga rectifier in shunt with the sensitivity resistor. This rectifier isconnected so as to provide a low impedance circuit to the capacitorenabling the capacitor to charge rapidly when the unit is .turned on;and a high impedance path to the discharge of the capacitor so that thecapacitor discharges through the sensitivity resistor which remainseffective to prevent false alarms. I

Another advantage of the present alarm circuit is that it facilitatesthe use of a standard relay eliminating the need for an expensive meterrelay of the type previously required. The elimination of a meter typerelay not only renders the system more economical, but it also resultsin a system having a more rapid response than was previously possible orpractical.

These and other objects and advantages of the present invention will bemore readily apparent from the following detailed description of thedrawing illustrating a preferred embodiment of the invention.

In the drawing:

FIGURE 1 is a diagrammatic view showing the manner in which a capacityalarm system of the present type is installed to protect ofliceequipment.

FIGURE 2 is an electrical wiring diagram of the pertinent portion of thepresent capacity alarm system.

The present alarm system is particularly adapted for the protection ofstorage units in offices, stores and the like, although it will readilybe appreciated that the unit has utility in other installations as well.One typical installation of the present system is shown in FIGURE 1. Asis there shown, the objects to be protected include a file cabinet 10and a safe 11. These units are electrically connected to one another asindicated by the connector strip 12. Both the file cabinet 10 and safe11 are insulated from ground as indicated diagrammatically by insulationblocks 16, 14 and 15.

The present alarm system includes an oscillator circuit and a detectorcircuit which are connected through a lead 16 to the safe 11 and filecabinet 10. These latter units, which are connected in series, form anantenna system for the present alarm. The oscillator circuit iseffective to radiate radio frequency energy through the resonant antennainto the area surrounding the antenna system. Thus, an electro magneticfield of stored low radio frequency energy is set up in the area of theroom surrounding the safe and file cabinet. The electrical circuit alsoincludes a high-Q detector coil which is coupled to the oscillator andtuned to the frequency of the oscillator.

The detector circuit is tuned so that any increase in capacity to groundof the resonant circuit will cause a voltage drop across the coil.Consequently, as a person approaches the protected units, such ascabinet and safe 11, he increases the capacity of the resonant circuitand causes a voltage drop in the detector circuit which is ultimatelyutilized to actuate relays causing the energization of either a localalarm, a remote alarm or a combination of the two.

In the system illustrated in FIGURE 1, a power supply, oscillatorcircuit, detector circuit and relay for controlling a remote alarm andlocal alarm (not shown), are mounted in an electrically conductivehousing 17 which itself preferably forms part of the antenna system.

This housing is interconnected through a suitable cable 18 to a remotealarm 19.

More particularly, as is shown in FIGURE 2, the capacity alarm comprisesa power source, such as batteries 20, a low radio frequency oscillatorcircuit 21, a detector stage 22 and a relay amplifier stage 23. Therelay amplifier stage 23 controls energization of a relay coil 24 whichin turn effects actuation of remote alarm 19. It is to be understoodthat relay 24 can likewise be used to control energization of a localalarm, such as a bell (not shown), located in or near the area beingprotected.

As is shown in FIGURE 2, the positive terminal of battery is connectedto a grounded common line 25 while the negative terminal of the batteryis connected to line 26. A capacitor 27 is shunted across these lines.Line 26 is joined to the movable contact 2 8 of a threeposition switch30. Switch 30 is a day-night switch and is provided with a night contact31, an open day" contact 32 and a test contact 33. It is also to beunderstood that in practice day-night switch 30 is a multi deck switchand that the decks (not shown) are utilized in conjunction with timercircuits, test circuits, and the like, which have been omitted from thepresent schematic diagram since these circuits constitute no part of thepresent invention.

Both contacts 31 and 33 of switch 30 are joined to line 34. A lead 35 isconnected to line 34. This lead is connected to base 36 of oscillatortransistor 37 through resister 38. A capacitor 40 is shunted betweenlead 35 and common line 25. Transistor 37 also includes an emitter 41connected to common line 25 and a collector 42 joined to win-ding 43 ofoscillator transformer 44. The opposite terminal of winding 43 isconnected to base 36 through capacitor 45. The secondary winding 46 andtank capacitor 47 form the oscillator tank circuit and determine the RFoutput of the oscillator circuit. The output frequency of this circuitis preferably in the low radio frequency range, for example 15 kc.

One terminal of secondary winding 46 is tied to common line 25 throughlead 48, while the opposite terminal of this winding is connectedthrough coupling capacitor 50 to the antenna tunning circuit whichincludes tuning capacitor 51 connected in parallel with the primarywinding 52 of detector transformer 53. Specifically, one lead of winding52 is connected to common line 25 while the other lead is connectedthrough lea-d 54 to capacitor 50. The antenna system includes cabinets10 and '11 which are connected through lead 16 to an intermediate tap 55of primary winding 52 and housing 17 of the alarm unit which isconnected to primary winding 52 through lead 56 and capacitor 57. Thus,the alarm circuitry and the local alarm (if mounted within housing 17)are protected since the housing 17 forms part of the antenna system.

Detector transformer 53 further includes a secondary winding 58 which isconnected to common line 25 and through inductance 60 to base 61 ofdetector transistor 62. A capacitor 63 is shunted between base 61 andcommon line 25. Detector transistor 62 includes a collector 64 connectedthrough lead 65 to line 34. The emitter 66 of transistor 62 is connectedthrough resistor 67 to a milliammeter 68, the other terminal of themilliammeter being connected to common line 25. Emitter 66 is connectedthrough resistor 70 to 'base 7.1 of relay amplifier transistor 72. Adiode rectifier 80 and coupling capacitor 81 are connected in serieswith one another and are shunted across resistor 70. Asensitivity'resistor 82 is in turn shunted across diode rectifier 89. Inone circuit, resistor 70 is 22K ohms, resistor 82 is 5.1K ohms andcapacitor 81 is 3000 microfarads. A filter capacitor 73 is shuntedbetween emitter 66 and common line 25. Emitter 74 of transistor 72 isconnected through lead 75 to common line 25. The collector 76 oftransistor 72 is connected through one lead of relay coil 24. Theopposite end of this relay coil is joined to line 34 and the relay coilis shunted by a capacitor 77.

Relay 24 includes a movable contact 83 engageable with either fixedcontact point 84- or 85. The relay is shown in an energized condition inFIGURE 2. With the relay in this condition, movable contact 83 is in engagement with lower stationary contact 85. When the relay is'deenergized, movable contact 83 is shifted into engagement with upperstationary contact 84, completing a circuit to actuate remote alarm 19and a local alarm (not shown).

In operation, the detector coil 52 is tuned so that any increase incapacity to ground in the vicinity of the antenna system, i.e. cabinets10 and 11 and casing 17, will cause a drop in voltage developed acrosswinding 58. This is accomplished by tuning the coil from minimumcapacity through the resonant peak and slightly beyond it. Consequently,if a person approaches the protected objects, he causes an increase inthe capacity relative to earth ground of the tuning circuit with theresultant drop of voltage across winding 58. across the detector windingis rectified and is A.C. coupled to the transistor relay stage 23.Consequently, any drop in rectified voltage will cause relay 24 to dropout, energizing the remote and local alarms.

Specifically, the rectified -D.C. voltage, which is of the order of .70v. with respect to ground, appears at the emitter 66 of transistor 62.Capacitor 73 acts as a filter for. this voltage. This voltage is A.C.coupled through resistor 82 to the relay amplifier stage 23, L6. to thebase of transistor 72. This transistor is in a saturated or full onstate so that practically all of the supply voltage is developed acrossthe relay coil 24 in the collector circuit of the transistor 72.

It will be appreciated that relay amplifier transistor 72 receives itsDC. bias through resistor 70 interconnecting base 71 of transistor 72and emitter 66 of transistor 62. If this bias voltage is insufiicient,as for example because of the excessive tuning of the detector coil orlow supply voltage, transistor 72 will not be saturatedrand relay 24will not be energized to cause an alarm. Thus, this portion of thecircuit functions as a fail-safe circuit which has the efiect of being aconstant monitor of the detector tuning, oscillator voltage, batteryvoltage and of the circuit components which might be defective. It willreadily be appreciated that if transistor 72 were biased directly frombattery 20, a slow drop in detector voltage to an insensitive statecould happen Without triggering an alarm. Moreover, turning on the alarmwith the detector detuned or the oscillator inoperative would also failto cause an alarm.

Capacitor 81 is a coupling capacitor which is effec- This voltagedeveloped tive to provide a low impedance coupling path for shorttermchanges. Specifically, when the capacitor alarm is first turned on andproperly balanced, a D.C. voltage appears at the emitter 66 oftransistor 62. Transistor 72 being biased from this point begins to drawbias current through a circuit from its emitter 74 through its base 71,and resistor 70 to the emitter 66 of transistor 62.

Since capacitor 81 must also be charged to approximately -.70 volt,additional current flows from the emitter of transistor 72 through itsbase, rectifier 80, and resistor 82 to the capacitor 81.

After capacitor 81 becomes charged, the only current continuing to flowbetween stages 22 and 23 will be bias current flowing through resistor70. Any slow, long-term changes in the detector voltage appearing atcollector 66 will cause a very slow charge or discharge of couplingcapacitor 81, but this will not cause any appreciable change in thesaturated state of transistor 23. However, any rapid decrease in thedetector voltage causes a likewise rapid discharge of capacitor 81. Thiscapacitor discharges through resistor 82 and through the base emittercircuit of transistor 72. Since this discharge current is in theopposite direction from the bias current flow from the emitter 74 oftransistor 72 to the emitter 66 of transistor 62, the net bias currentis reduced depending upon the rate at which capacitor 81 discharges. Ifthe bias current is reduced by a suflicient amount, transistor 72 willno longer be saturated and will not pass enough current through itscollector circuit to keep relay 24 energized. When this relay isdeenergized, contact 83 shifts into engagement with stationary contact84 causing actuation of remote alarm 19 and the local alarm.

Resistor 82 serves as a desensitizing resistor. Specifically, thisresistor reduces the coupling between detector transistor 62 andamplifier transistor 72 and limits the frequency response of the alarm.This resistor further inherently tends to limit the charge up rate of capacitor 81 which adversely affects the sensitivity of the alarm unitwhen it is first turned on. Specifically, as long as capacitor 81 ischarging, it is drawing bias current through transistor 72.Consequently, a large drop in detector voltage is required to cause thecapacitor to start discharging through transistor 72 to render itnonconductive.

In accordance with the present invention, a prolonged loss insensitivity is avoided and the time of less than maximum sensitivity isminimized by diode 80. Diode 80 effectively short circuits the resistor82 during charge up because it is forward conducting, i.e. provides alow impedance path to capacitor 81. Capacitor 81 thus charges up tomaximum potential very rapidly and provides a fast set up time for thealarm as far as sensitivity is concerned. By way of example, the presentalarm is set up in approximately one second as compared to the fortyseconds required by previous units. Nevertheless, when the unit is inoperation, resistor 82 is still effective to limit the sensitivity ofthe device as it was intended. More particularly, an alarm is caused bythe discharge of capacitor 81 in a reverse direction in which rectifier80 is nonconductive. Thus, the entire discharge current of capacitor 81must flow through resistor 82.

From the foregoing disclosure of the general principles of the presentinvention and the above description of a preferred embodiment, thoseskilled in the art will readily comprehend the various modifications towhich the invention is susceptible. Therefore, we desire to be limitedonly by the scope of the following claims.

Having described our invention, we claim:

1. A capacity alarm system comprising an oscillator, a tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means, a detector for sensing changes in capacitance in saidtuned resonant circuit and developing a D.C. potential in responsethereto, amplifier means, fail-safe circuit means interconnecting saiddetector and said amplifier means, said fail-safe circuit meansincluding means providing a conductive path for D.C. operating bias fromsaid detector for said amplifier means, whereby said amplifier output issubstantially reduced in response to a gradual change in detector outputin excess of a predetermined magnitude, said fail-safe circuit alsoincluding a capacitor providing an A.C. coupling between said detectorand amplifier means, said capacitor being discharged in response to arapid change in detector output and being effective to substantiallyreduce the output of said amplifier means, and alarm means energized bysaid amplifier means and adapted for actuation in response to saidsubstantial reduction in the output of said amplifier means.

2. A capacity alarm system comprising an oscillator, a tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means, a detector for sensing changes in capacitance in saidtuned resonant antenna circuit and developing a D.C. potential inresponse thereto, amplifier means for controlling actuation of an alarm,and fail-safe circuit means interconnecting said detector and saidamplifier means, said failsafe circuit comprising a resistorinterconnecting said detector and said amplifier means to provide a D.C.operating bias for said amplifier means, and parallel circuit meansincluding a capacitor effective to cause current flow in a reversedirection to said amplifier in response to an increase in capacity insaid resonant circuit and a decrease in potential of said detector.

3. A capacity alarm system comprising an oscillator, a tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means, a detector for sensing changes in capacitance in saidtuned resonant antenna circuit and developing a D.C. potential inresponse thereto, amplifier means for controlling actuation of an alarm,and fail-safe circuit means interconnecting said detector and saidamplifier means, said fail-safe circuit comprising a resistorinterconnecting said detector and said amplifier means to provide a D.C.operating bias for said amplifier means, and parallel circuit meansincluding a capacitor and series connected sensitivity resistoreffective to cause current flow in a reverse direction to said amplifierin response to an increase in capacity in said resonant circuit and adecrease in potential of said detector.

4. A capacity alarm system comprising an oscillator, 21 tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means, a detector for sensing changes in capacitance in saidtuned resonant antenna circuit and developing a D.C. potential inresponse thereto, amplifier means for controlling actuation of an alarm,and fail-safe circuit means interconnecting said detector and saidamplifier means, said fail-safe circuit comprising a resistorinterconnecting said detector and said amplifier means to provide a D.C.operating bias for said amplifier means, and parallel circuit meansincluding a capacitor and series connected sensitivity resistoreffective to cause current flow in a reverse direction to said amplifierin response to an increase in capacity in said resonant circuit and adecrease in potential of said detector, and a diode shunting saidsensitivity resistor, said diode being connected to provide a highimpedance path to current discharged by said capacitor.

5. A capacity alarm system comprising an oscillator, a tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means and an inductive winding, a first transistor connected tosaid inductive winding for sensing changes in capacitance in said tunedresonant antenna circuit, said transistor including a collector having aD.C. potential in variable response to said change in capacitance, asecond transistor for controlling actuation of an alarm, and fail-safecircuit means interconnecting said first transistor and said secondtransistor, said fail-safe circuit comprising a resistor interconnectingsaid collector and said second transistor to provide a D.C.

operating bias for said second transistor, and parallel circuit meansincluding a capacitor effective to cause current flow in a reversedirection to said second transistor in response to an increase incapacity in said resonant circuit and a decrease in potential of saidcollector.

6. A capacity alarm system comprising an oscillator, a tuned resonantcircuit connected to said oscillator, said resonant circuit includingantenna means and an inductive winding, a first transistor connected tosaid inductive winding for sensing changes in capacitance in said tunedresonant antenna circuit, said transistor including a collector having aD.C. potential in variable response to said change in capacitance, asecond transistor for controlling actuation of an alarm, and fail-safecircuit means interconnecting said first transistor and said secondtransistor, said fail-safe circuit comprising a resistor interconnectingsaid collector and said second transistor to provide a D.C. operatingbias for said second transistor, and parallel circuit means including acapacitor and series connected sensitivity resistor effective to causecurrent flow in a reverse direction to said second transistor inresponse to an increase in capacity in said resonant circuit and adecrease in potential of said collector.

7. A capacity alarm system comprising an oscillator, a tune-d resonantcircuit connected to said oscillator, said resonant circuit includingantenna means and an inductive winding, a first transistor connected tosaid inductive winding for sensing changes in capacitance in said tunedresonant antenna circuit, said transistor including a collector having aD.C. potential in variable response to said change in capacitance, asecond transistor for controlling actuation of an alarm, and fail-safecircuit means interconnecting said first transistor and said secondtransistor, said fail-safe circuit comprising a resistor interconnectingsaid collector and said second transistor to provide a D.C. operatingbias for said second transistor, and parallel circuit means including acapacitor and series connected sensitivity resistor effective to causecurrent flow in a reverse direction to said second transistor inresponse to an increase in capacity in said resonant circuit and adecrease in potential of said collector, and a diode shunting saidsensitivity resistor, said diode being connected to provide a highimpedance path to current discharged by said capacitor.

References Cited by the Examiner FOREIGN PATENTS 668,374 3/1952 GreatBritain.

NEIL C. READ, Primary Examiner.

R. GOLDMAN, Assistant Examiner.

7. A CAPACITY ALARM SYSTEM COMPRISING AN OSCILLATOR, A TUNED RESONANTCIRCUIT CONNECTED TO SAID OSCILLATOR, SAID RESONANT CIRCUIT INCLUDINGANTENNA MEANS AND AN INDUCTIVE WINDING, A FIRST TRANSISTOR CONNECTED TOSAID INDUCTIVE WINDING FOR SENSING CHANGES IN CAPACITANCE IN SAID TUNEDRESONANT ANTENNA CIRCUIT, SAID TRANSISTOR INCLUDING A COLLECTOR HAVING AD.C. POTENTIAL IN VARIABLE RESPONSE TO SAID CHANGE IN CAPACITANCE, ASECOND TRANSISTOR FOR CONTROLLING ACTUATION OF AN ALARM, AND FAIL-SAFECIRCUIT MEANS INTERCONNECTING SAID FIRST TRANSISTOR AND SAID SECONDTRANSISTOR SAID FAIL-SAFE CIRCUIT COMPRISING A RESISTOR INTERCONNECTINGSAID COLLECTOR AND SAID SECOND TRANSISTOR, AND A PARALLEL OPERATING BIASFOR SAID SECOND TRANSISTOR, AND A PARALLEL CIRCUIT MEANS INCLUDING ACAPACITOR AND SERIES CONNECTED SENSITIVITY RESISTOR EFFECTIVE TO CAUSECURRENT FLOW IN A REVERSE DIRECTION TO SAID SECOND TRANSISTOR INRESPONSE TO AN INCREASE IN CAPACITY IN SAID RESONANT CIRCUIT AND ADECREASE IN POTENTIAL OF SAID COLLECTOR, AND A DIODE SHUNING SAIDSENSITIVITY RESISTOR, SAID DIODE BEING CONNECTED TO PROVIDE A HIGHIMPEDANCE PATH TO CURRENT DISCHARGED BY SAID CAPACITOR.